A ferritic stainless steel has been known as a raw material in manufacturing a wire to be used for gas shielded arc welding. Ferritic stainless steels are low-priced compared with austenite stainless steels, and are not likely to cause heat fatigue since they has low heat expansion efficient. A weld metal formed by using such wire (a metal of a welded portion where a welded base material and a welding material or welding wire have been mixed) exhibits excellent cracking resistance in the presence of a chloride. In some cases where a ferritic stainless steel is used as a base material in welding, a small amount of martensite is formed by a component such as Cr (Chromium) in a portion of the base material that is thermally affected by welding. Most portions of the base material, however, remain ferritic. Since the ferritic stainless steel is hardly deformed, the formation of martensite is increased in the weld metal when an amount of heat becomes large during welding. As a result, the martensite crystals are coarsened, thereby weakening the weld metal.
When a ferritic stainless steel is used as a base material and the same material (or a material having a similar composition to that of the base material) is used as a welding wire the foregoing also applies to a weld metal which is formed by welding the base material. Due to the presence of coarsened crystals, the weld metal has low tensile strength, and is also low in toughness as measured by Charpy impact test and flexural property. In addition, the cracking resistance of the weld metal is also decreased. Although the tensile strength can be increased by heat treatment after welding, the toughness as measured by Charpy impact test and flexural property cannot be improved even after the heat treatment.
As is shown in Japanese Patent Application Laid-open Publication No. 2004-42116, Japanese Patent Application Laid-open Publication No. 2004-141934, Japanese Patent Application Laid-open Publication No. 2005-971, and Japanese Patent No. 2500008, a ferritic stainless steel wire having high cracking resistance has been proposed whereby coarsening of weld metal crystals is prevented by adding a small amount of Al (Aluminum) and Ti (Titanium).
In the specification of the present invention, “high cracking resistance” means that the beads (weld metal) are not readily cracked after welding, and “low cracking resistance” means that the weld metal is readily cracked after welding. “High flexural property” means that the weld metal is not readily cracked when the metal is bent and “low flexural property” means that the weld metal is readily cracked when the metal is bent. “High corrosion resistance” means that the weld metal is not readily corroded and “low corrosion resistance” means that the weld metal is readily corroded. “High oxidation resistance” means that the weld metal is not readily oxidized and “low oxidation resistance” means that the weld metal is readily oxidized.
Especially, the following requirements are necessary for a ferritic stainless steel welding wire for use in welding parts of an automobile exhaust system.
(1) Prevention of wire breaking and ease of manufacturing
(2) High cracking resistance of the weld metal
(3) Large grain number and small grain size of the weld metal
(4) High flexural property of the weld metal
(5) Tensile strength of the weld metal up to high temperature above the standard which is required for the base material
(6) High corrosion resistance of the weld metal
(7) High oxidation resistance of the weld metal
The conventional ferritic stainless steel wires do not satisfy all of these requirements. Usually, welding of parts used in an automobile exhaust system are efficiently done by automatic equipment such as a robot. In these circumstances, welding must be stabilized by using a wire having adequate hardness and smooth surface and supplying the wire smoothly.